Preheat oven to 350°F. Line two cookie sheets with parchement. Using a standing mixer with paddle, cream the butter, sugar, and light brown sugar until fluffy and light. Add the eggs one a time while beating on low, stopping to scrape down the sides and blade as needed.
Add thecorn syrup and vanilla and beat until light and fluffy again. In a medium bowl, whisk the flour, cocoa powder, cornstarch, salt, and baking soda. Gradually beat the dry ingredients into the butter on low until it comes together.
Using a 1/4 cup measuring cup, scoop out dough mound into your hand and roll into a ball. Place onto the cookie sheet leaving a lot of space because these spread out. I did 4 per sheet.
Bake in the preheated oven for 12-14 minutes or until firm around the edge and not raw looking in the center. Allow to cool for a few minutes on the baking sheet. Then move the cookies to a cooling rack to cool completely.
For the frosting and topping:
Use a standing mixer with paddle attachment, beat the butter, powdered sugar, and vanilla until creamy and well mixed. Add cream as needed to thin the icing a little. Scoop the frosting into the piping bag. Pipe a tight spiral onto the top of the cookie. Sprinkle chopped heath bars on top.